Molten metal heat-exchanger

ABSTRACT

The molten metal heat-exchanger for heating water comprises a recipient containing a metal with a low melting point in its interior, with a tube immersed in the metal and in the interior of the recipient through which the water to be heated circulates. To heat the water a heat source is applied to the recipient heating the metal therein until it changes to a liquid state. If the temperature of the metal in the liquid state is lower than the boiling temperature of water, the exchanger produces hot water, whereas if the temperature of the liquid metal is higher than the boiling temperature of water, the exchanger can produce hot water or steam. The metal to be used in the molten metal heat-exchanger is a metal or metal alloy with a melting temperature of close to 100° C., with gallium, which has a melting temperature of 30° C. and a boiling temperature of 2204° C., being ideal for the application, although currently existing and future metal alloys may be used in the molten metal heat-exchanger.

DESCRIPTIVE MEMORY Description of Field Knowledge

Use of Heat Exchangers in Boilers.

Central heating raises the temperature of an entire building using only one heating point, which is usually a boiler. Every boiler has at least: a source of caloric energy, a fluid—most often water—that is circulated through the building by a pump, and a heat exchanger.

During heat transfer, 3 processes occur in a heat exchanger:

-   -   Conduction, convection, and radiation.

Problems Detected in Heat Exchangers

Radiation Losses (Distance from the Energy Source to the Exchanger) in boilers that use solid or liquid fuels, the exchanger mainly heats the water through convection from the hot gases of a flame. The usable radiation for the exchanger varies inversely with the square of the distance that separates it from the flame so in a boiler, the heat exchanger is moved away from the flame to avoid damage due to excessive temperatures. As a result, the contribution to radiation heating is relatively low.

Thermal oil is also used as a heat transfer fluid, either to heat water or as a single heat carrier. If the thermal oil comes into contact with hot spots, it is degraded by the breakage of its molecules. Therefore, when using thermal oil, the heat exchanger must be moved away from the flame.

Some boiler designs circulate the heat transfer fluid in order to take advantage of radiation heating from the heat source, but the burner design itself sets the minimum distance at which radiation can be harnessed.

During combustion inside of a boiler, the flame temperature is much higher than the temperature of the exhaust gases. Therefore, it would be beneficial to be able to take better advantage of the high temperatures generated by the flame by bringing the heat exchanger closer to the flame.

In summary, part of the energy emitted by the flame inside of a boiler is lost because of the need to move the flame away from the heat exchanger.

Condensation

As the exhaust gases in a boiler are in contact with the heat exchanger tubes that carry water, the boiler must have a minimum inbound temperature. If the pipes are cooled by a low water temperature, condensation of water vapor from the exhaust gases occurs, and this forms acids that cause corrosion inside the boiler.

Salts

Salts in the water cause problems in heat exchangers. The calcareous inlays, deposited inside the pipelines that carry the water, reduce the heat transmission between the combustion chamber and the water, and the plate does not cool properly. This produces surges in the material along with deformation and cracks in the tubes.

Maintenance

Heat exchangers heated by hot gases require thin walls. Furthermore, the fins used in some heat exchangers to improve heat transfer are also very thin and fragile by design. Both of these factors make maintenance and repair more difficult.

Variations in Water Flow

When the flow of water circulating through the heat exchanger varies, fluctuations occur in the temperature of the outbound water. This is because the time spent by the water inside the heat exchanger also varies, and this causes the energy absorbed by the water to increase or decrease.

Metallic Solids with Low Melting Points

Certain metals are distinguished by their low melting points. Two of these, lead and tin, are the most commonly used non-ferrous metals after copper and aluminum.

The most common applications for low-melting metals are soft solders and low-melting alloys. Tin, on the other hand, is mainly used as a coating for anti-corrosion steel.

A wide range of sodium and potassium alloys are liquid at room temperature, and they are used as refrigerants.

Because of their stability, low-melting metals are used in cooling systems to extract heat in high temperature areas. Examples include: WO 2007115827 A1; EP 1844880 A1 (20074); U.S. Pat. No. 8,789,377 B1 (2014); U.S. Pat. No. 3,129,754 A (1964).

Due to their thermal stability, gallium and indium alloys are used as a means to control temperature in biological samples such as WO 2000069561 A1.

In general, low-melting metal solids in heat transfer systems are used as heat carriers to extract heat from hot areas.

Use of Solids in Steam Generation

A memory request submitted in Chile and later abandoned, proposed a solid block with pipes inside made of materials with high thermal conductivity as a heat exchanger. Since the block was in contact with the exhaust gases, it would transfer the heat to the tubes to produce steam, 200003589.

DESCRIPTION OF THE FIGURES

In addition to the detailed description provided, there are two figures attached that schematically represent the implementation of the invention using a spiral tube. While other designs are possible to improve efficiency or facilitate maintenance, the figures represent a configuration that facilitates the explanation of the invention as an example and without any limiting factors.

FIG. 1: Perspective projection of the implementation of the invention using a spiral tube.

FIG. 2: Sectional projection of the implementation of the invention using a spiral tube.

There is no restriction of the direction water flows into the heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION

The present invention proposes a heat exchanger to generate hot water which, among other advantages, prevents condensation of exhaust gases and prevents structural damage from salt accumulation. It also has a robust shockproof construction, maintains the temperature of the outbound water within a range of just a few degrees, occupies minimal space, and has a better performance because it allows for the least possible distance between the flame of a burner and the heat exchanger.

In order to heat the water, the invention proposes a heat exchanger composed of a container with a low melting metal inside as well as a pipe through which the water that will be heated circulates. Then, to heat the water, a heat source is applied to the container that heats the metal inside, transforming it into its liquid state. If the temperature of the liquid metal is lower than the boiling temperature of the water, the exchanger produces hot water; if the liquid metal temperature is higher than the boiling temperature of water, the exchanger may produce hot water or steam. The heat exchanger described is called a molten metal heat exchanger.

To heat water, the invention proposes a molten metal heat exchanger composed of:

-   -   1.—A container (1) that has at least two orifices.     -   2.—A pipe (2) for the water to flow so that the water enters the         said container through one of the two orifices and exits through         the other orifice.     -   3.—Tight seals (3) in each one of the orifices between the pipe         and the container.     -   4.—A metal (4) inside said container that covers part or all of         the pipe inside the container as said metal was poured into the         container when it was in its liquid state.     -   5.—Means of connection (5) in the pipes that allow the use of         the heat exchanger and other hot water generating equipment in         the boilers.     -   6.—Heatsource(6).

The metal or metal alloy used in the molten metal heat exchanger needs to have a melting temperature of less than 100° C. For example, gallium is an ideal metal for this as it has a melting temperature of 30° C. and a boiling temperature of 2204° C. Nevertheless, existing metal alloys, as well as metal alloys yet to be developed, would work as well in the molten metal heat exchanger.

In this case, the container and pipes are made of metal; however, stainless steel, among other materials, may also be used as long as they withstand operating temperatures.

Operating Mode

The correct operation of the molten metal heat exchanger requires:

-   -   A heat source (6) that heats the container until the metal melts         inside, and the temperature inside the molten metal is         homogenized by convention.     -   A pump that drives water to pass through the molten metal heat         exchanger.     -   A control system that measures temperatures and manages the         water flow and the heat source power. The temperatures can be         measured of the inbound water, the outbound water, and the         molten metal.

One possible way to control the operation of the molten metal heat exchanger is to set the temperature of the molten metal, for example, at 90° C. and establish that the hot water temperature will be maintained within a range of 5° C. below that temperature. This can be achieved by varying the flow of water and the power of the heat source. As the response of the molten metal heat exchanger to the variation of parameters is already installed, the control system will read the inbound temperature of the water entering the vessel, and it will regulate the flow rate and the caloric power delivered to the vessel to conserve the temperature of the molten metal and maintain the temperature of the outbound water in the preset range. In its daily use, the power consumed by the molten metal heat exchanger is a function of the inbound water temperature.

Other forms of control are possible.

The terms in which this detailed description of the invention has been written should always be taken in a broad and non-limiting sense.

Advantages of the Molten Metal Heat Exchanger.

Radiation Losses (Distance from the Energy Source to the Exchanger).

Radiation losses are reduced, and the molten metal heat exchanger can be placed very close to, if not directly on the flame.

Condensation.

The exhaust gases in a molten metal heat exchanger are not in contact with the water-filled pipes. Therefore, although the temperature of the inbound water is very low, condensation of the water vapor in the exhaust gases cannot occur. This prevents the emergence of acids and the subsequent corrosion.

There are no restrictions on the temperature of the inbound water.

Salts.

In a molten metal heat exchanger, even though the plate that forms the tubes has calcareous inlays, it is submerged in a liquid that is only a few tens of degrees above the temperature of the water inside. Thus, the thermal gradient is not enough to produce structural damage.

Maintenance.

At room temperature, the molten metal heat exchanger is a solid block with pipes, and it is very resistant to impacts and other handling. Additionally, if the design is suitable, the interior of the pipes can be easily cleaned by mechanical means.

If there is ever a lack of water in the molten metal heat exchanger, there are no breakdowns in the equipment due to the fact that there are no hot spots left without cooling mechanisms in place. In fact, the molten metal itself acts as a temperature regulator.

Variations in the Heat Transfer Flow Rate.

The maximum temperature that water can reach in a molten metal heat exchanger is the temperature of molten metal; in other words, its temperature can only reach a few degrees higher than the outbound water temperature.

Efficiency.

In addition to making better use of the flame in the heat source, thermal energy is also better utilized as metals have a lower specific heat and greater thermal conductivity than air and water.

For example, at room temperature, to increase the temperature of one liter of gallium by one degree, less than half the energy needed to increase the temperature of one liter of water is needed.

Due to the high thermal conductivity of metals, heat inside the molten metal heat exchanger is transported to the pipe through natural convection and conduction, but the transfer can also be increased through forced convection by means of some type of agitator. As a turbulent flow ensures high convection, stirring the molten metal does not require great power because at the working temperature, the viscosity of low melting metals is similar to the viscosity of water. 

1.- A heat exchanger for heating water, IS CHARACTERIZED as such because it is comprised of: a container (1) with at least two orifices; a pipe (2) for the flow of water that enters into said container through one of the orifices and out through the other orifice; a tight connection (3) in each of the orifices between the pipe and the container; a metal (4) inside said container that covers part or all of the pipe inside the container as said metal was poured into the container when it was in its liquid state; and a means of connection (5). 2.- According to claim 1, a heat exchanger for heating water IS CHARACTERIZED as such because the metal is a pure metallic solid or a low-melting metal alloy that remains in a liquid state during operation of the heat exchanger. 3.- According to claim 1, a heat exchanger for heating water IS CHARACTERIZED as such because the metallic solid is gallium or a metal alloy containing it. 4.- According to claim 1, a heat exchanger for heating water IS CHARACTERIZED as such because the molten metal has a lower temperature than the boiling temperature of the water at one unit of atmospheric pressure, and the water passing through the heat exchanger does not change phase. 5.- According to claim 1, a heat exchanger for heating water IS CHARACTERIZED as such because the metallic solid is at a temperature higher than the boiling temperature of the water at one unit of atmospheric pressure, and the water when passing through the heat exchanger does not change phases. 6.- According to claim 1, a heat exchanger for heating water IS CHARACTERIZED as such because the metallic solid is melted at a temperature higher than the boiling temperature of the water at one unit of atmospheric pressure, and the water passing through the heat exchanger changes phases. 7.- According to claim 1, the process for heating water IS CHARACTERIZED as heating water when it flows through the pipe that is submerged in the molten metal inside the container while it is at a temperature below the boiling temperature of the water at one unit of atmospheric pressure. 8.- According to claim 1, the process for heating water IS CHARACTERIZED as heating water when it flows through the pipe that is submerged in the molten metal into the container while it is at a temperature higher than the boiling temperature of water at one unit of atmospheric pressure. 9.- According to claim 1, the process for producing water vapor, IS CHARACTERIZED as heating water when it flows through the pipe that is submerged in the molten metal inside the container while the molten metal is at a temperature higher than the boiling temperature of the water at one unit of atmospheric pressure. 